One type of electrostatographic reproducing machine is a xerographic copier or printer. In a typical xerographic copier or printer, a photoreceptor surface, for example that of a drum, is generally arranged to move in an endless path through the various processing stations of the xerographic process. As in most xerographic machines, a light image of an original document is projected or scanned onto a uniformly charged surface of a photoreceptor to form an electrostatic latent image thereon. Thereafter, the latent image is developed with an oppositely charged powdered developing material called toner to form a toner image corresponding to the latent image on the photoreceptor surface. When the photoreceptor surface is reusable, the toner image is then electrostatically transferred to a recording medium, such as paper, and the surface of the photoreceptor is cleaned and prepared to be used once again for the reproduction of a copy of an original. The paper with the powdered toner thereon in imagewise configuration is separated from the photoreceptor and moved through a fuser apparatus to permanently fix or fuse the toner image to the paper.
Typically, a fuser apparatus of the type provides a combination of heat and pressure to fix the toner image on the paper. The basic architecture of a fuser apparatus is well known. Essentially, it comprises a pressure roll that forms a fusing nip against a rotatable heated that is heated internally or externally as disclosed for example in U.S. Pat. No. 6,895,207 issued May 17, 2006. A sheet of paper carrying an unfused or powder toner image is passed through the nip. The side of the paper having the unfused or powder toner image typically faces the fuser roll, which is often supplied with a heat source, such as a resistance heater, at the core thereof. The combination of heat from the fuser roll and pressure between the fuser roll and the pressure roll fuses the toner image to the paper, and once the fused toner cools, the image is permanently fixed to the paper.
Conventional fusing apparatus ordinarily only having single or dual heating elements and thus suffer from lack of precise axial thermal uniformity, particularly in fusing apparatus being required to run at relatively higher and higher throughput speeds, and those using copy sheets or media of various sizes. The problem is even more pronounced in color image fusing apparatus that include fusing members having relatively thicker silicone or rubber outer layers for providing the conformance and dwell time necessary for the increased toner pile heights of color images. In such fusing apparatus, the axial thermal uniformity of the fusing member will get worse as the rubber thickness increases and also as the lengths of jobs vary. In addition, if a paper is run that is more narrow than the heater element, hot spots on the outboard edges are formed resulting in inconsistent fix and gloss.
Examples of conventional fusing apparatus can be found in U.S. Pat. No. 6,895,207 issued May 17, 2006 as mentioned above, and in U.S. Pat. No. 6,407,366 issued Jun. 18, 2002 and entitled “Image heating apparatus having a plurality of heat generating elements”. For the purpose of having only a small number of semiconductor switching elements, this reference discloses long heating elements that are treated similar to lamps in that they are multiple long elements parallel to the long axis, and turned on and off like lamps depending on whether the job runs on letter size or legal size sheets.
U.S. Pat. No. 6,734,397 issued May 11, 2004 and entitled “Heater having at least one cycle path resistor and image heating apparatus therein” discloses a heater, or an image heating apparatus including a heater that has a substrate, heat generating resistors formed at least in a cycle path on the substrate, and current supply electrodes provided at electrical ends of the heat generating resistors, wherein plural heat generating resistors are connected in parallel to at least one of the current supply electrodes. Thus there can be obtained a heater having excellent heat generating characteristics even in a compact dimension and an image heating apparatus utilizing such heater.